A handful of U.S. intelligence agencies have advised American citizens not to buy smartphones made by Chinese tech firms.

Six top intelligence officials, including the directors of the FBI, CIA, NSA and Defense intelligence Agency, gave the stern warning when testifying to the Senate Intelligence Committee on Tuesday, CNBCreported.

The officials suggested that devices manufactured by firms such as Huawei or ZTE may pose a security threat to U.S. citizens who purchase them. Reportedly, a senator asked the agency chiefs to raise their hands if they recommended U.S. customers buy or use these products. None of them did, according to CNN Tech.

There’s a risk when letting any firm “beholden to foreign governments” into U.S. telecom networks, FBI Director Chris Wray explained during his testimony.

“That provides the capacity to exert pressure or control over our telecommunications infrastructure,” Wray added. “It provides the capacity to maliciously modify or steal information. And it provides the capacity to conduct undetected espionage.”

Huawei is the largest telecommunications equipment maker in the world, and more recently, it surpassed Apple as the world’s second-largest smartphone brand.

But while Huawei and other Chinese OEMs are dominant forces in their home market, they’ve mostly failed to gain any sort of traction in the U.S. Earlier this year, a plan to sell Huawei devices via AT&T fell through.

Largely, that’s because of U.S. government suspicion that the Chinese government could use smartphones and other devices made in China as espionage tools.

Congress has both introduced and passed legislation preventing government and federal agencies from buying from Chinese firms, or contracting them for other services. In 2012, Congress released a report saying Huawei and ZTE should be “viewed with suspicion,” CNN reported

For its part, Huawei said in a statement to CNBC that it “poses no greater cybersecurity risk” than any other tech vendor. Huawei CEO Richard Yu has also criticized American carriers and their “measured resistance” to Chinese firms, accusing them of diminishing customer choice.

It’s only the latest push by lawmakers to restrict foreign-made services or products due to intelligence concerns. Russia-based cybersecurity firm Kaspersky Lab, for example, has been banned from federal systems, CNN Tech reported.

[Update: ZTE has supplied an official statement to iDrop News 2/15/18] “ZTE is proud of the innovation and security of our products in the US market. As a publicly traded company, we are committed to adhering to all applicable laws and regulations of the United States, work with carriers to pass strict testing protocols, and adhere to the highest business standards. Our mobile phones and other devices incorporate US-made chipsets, US-made operating systems and other components. ZTE takes cybersecurity and privacy seriously and remains a trusted partner to our US suppliers, US customers and the people who use our high quality and affordable products for their communications needs.”